The term synthesis gas and syngas are frequently used to describe a mixture of gases prepared as feedstock for a chemical reaction. One example is a gas mixture of carbon monoxide and hydrogen which may be used as a feedstock or synthesis gas supplied to various reactions for making hydrocarbon compounds. Another example is a gas mixture of hydrogen and nitrogen which may be used as a feedstock or synthesis gas to make ammonia. A mixture of carbon monoxide and hydrogen is often used as a feedstock or synthesis gas for production of hydrocarbon compounds by a Fischer-Tropsch reaction which will be described later in more detail. U.S. Pat. No. 4,973,453, to Kenneth Agee, entitled Apparatus for the Production of Heavier Hydrocarbons from Gaseous Light Hydrocarbons, and U.S. Pat. No. 4,833,170, to Kenneth Agee, entitled Process and Apparatus for the Production of Heavier Hydrocarbons From Gaseous Light Hydrocarbons, provide information concerning synthesis gas and typical Fischer-Tropsch reactions. Both of these patents are incorporated by reference for all purposes.
A Fischer-Tropsch reaction is generally very exothermic and temperature sensitive. Therefore, temperature control is normally required to maintain a desired hydrocarbon product output. A Fischer-Tropsch reaction can be characterized by the following general formula: ##STR1## At relatively low to medium pressures (near atmospheric to 600 psig) and temperatures in a range from about 300.degree. F. to 600.degree. F., both saturated and unsaturated hydrocarbons can be produced. Numerous catalysts have been used in carrying out Fischer-Tropsch reactions.
Three basic methods have been employed for producing synthesis gas for use as a feedstock in a Fischer-Tropsch reaction. The methods are steam reforming wherein one or more light hydrocarbons such as methane gas are reacted with steam over a catalyst to form carbon monoxide and hydrogen, partial oxidation wherein one or more light hydrocarbons such as methane gas are combusted sub-stoichiometrically to form carbon monoxide and hydrogen and autothermal reforming which is a combination of steam reforming and partial oxidation. Steam reforming is normally an endothermic reaction. Partial oxidation and autothermal reforming normally are both exothermic reactions.
The steam reforming reaction of methane to produce synthesis gas may be represented by the following general formula: ##STR2##
A catalyst containing nickel is often utilized. The hydrogen to carbon monoxide ratio of the synthesis gas produced by steam reforming of methane is approximately 3:1.
Partial oxidation of methane to produce synthesis gas may be represented by the following general formula: EQU CH.sub.4 +1/2O.sub.2 .fwdarw.CO+2H.sub.2
Such partial oxidation is typically carried out using relatively high purity oxygen which may be expensive in comparison with other methods to form synthesis gas. A catalyst may or may not be used. The hydrogen to carbon monoxide ratio of the synthesis gas produced by partial oxidation of methane is approximately 2:1.
For some applications a combination of partial oxidation and steam reforming, known as autothermal reforming, may be used to produce synthesis gas. Air is generally used to provide oxygen for the associated partial oxidation reaction. U.S. Pat. No. 2,552,308 to F. J. Buchmann, et al., entitled Low-Pressure Hydrocarbon Synthesis Process, and U.S. Pat. No. 2,686,195 to D. R. McAdams, et al., entitled Hydrocarbon Synthesis, disclose low pressure hydrocarbon synthesis processes wherein autothermal reforming with air produces synthesis gas for a Fischer-Tropsch reaction. Both patents are incorporated by reference for all purposes. During an autothermal reforming reaction, the exothermic heat from the associated partial oxidation reaction may be used to provide heat required for the associated endothermic steam reforming reaction. An autothermal reforming process can be carried out in a relatively inexpensive refractory lined carbon steel vessel with generally lower costs as compared to partial oxidation in pure oxygen.
An autothermal reforming reaction typically produces synthesis gas with a lower hydrogen to carbon monoxide ratio than steam reforming. As previously noted, steam reforming of methane results in a ratio of about 3:1 while partial oxidation of methane results in a ratio of about 2:1. The optimum ratio for synthesis gas supplied to a Fischer-Tropsch reaction carried out at low to medium pressures over a cobalt based catalyst is approximately 2:1. When the feed input to an autothermal reforming process is a mixture of light hydrocarbons such as a natural gas with a relatively high methane content, additional controls are generally required to maintain the ratio of hydrogen to carbon monoxide in the synthesis gas produced by the autothermal reforming process at the optimum ratio of approximately 2:1.
Most of the currently available processes for producing synthesis gas from light hydrocarbons such as a natural gas also produce a residual gas stream. For some applications the residual gas stream may be used to provide energy to carry out the process of generating synthesis gas. For other applications the residual gas stream may be used to provide energy for further reactions to produce selected heavier hydrocarbons from the synthesis gas. Typically, several additional components and/or process steps are required for effective use of such residual gas.
It is desirable to generate synthesis gas for a Fischer-Tropsch reaction or any other reaction with as much thermal efficiency and at as low a cost as possible. The ability to develop an overall process with low capital expenses and low operating costs may be an imperative for development of effective commercial systems to produce relatively heavy hydrocarbons from lightweight gaseous hydrocarbons.